1. Field of the Invention
The present invention relates to image processing techniques, and more particularly, to methods and apparatus for non-linear dithering of images.
2. Description of the Prior Art
The bit numbers employed to express the respective pixels of an image, also known as the bit depth, determines the color depth of the image. In general, the visual quality of the image increases with the color depth.
The bit depth of the conventional display medium is limited; such as the computer monitor, the PDA display panel, and so forth. Conventionally, if the bit depth of the display medium is less than the color depth of the image to be displayed, then a technique is to be employed to display the image having more color depth on the display medium having less color depth. This technique, when applied to a display medium, is called dithering. In a printing context the same process is referred to as half toning. For example, when a 6 bits display medium is utilized to display an 8 bits image, the dithering technique is employed to display the image having more color depth on the display medium with lower bit depth.
Please refer to FIG. 1, which shows a simplified diagram describing a relationship between input intensity and output intensity of a display medium. The transverse axis of FIG. 1 is the input intensity of the display medium while the vertical axis is the output intensity of the display medium. Depending upon the type of the display medium, the input intensity may be the voltage of an electron gun, the input digital signal value of the display medium, etc. In FIG. 1, a point A corresponding to an input intensity 64 and a point B corresponding to an input intensity 68, are intensities that the display medium is capable of displaying without the need to utilize a dithering operation. The intermediate points corresponding to input intensities 65, 66, and 67, (i.e., the X points shown on FIG. 1), correspond to intensities that can be simulated by utilizing the dithering operations. As shown in FIG. 1, the X points are the dithered intensities obtained by the conventional dithering operations. The X points are located on a straight line defined by points A and B. In other words, the conventional dithering operations are linear dithering operations.
However, the conventional linear dithering operations cannot satisfy the requirements of many display media. FIG. 2 shows a diagram 200 describing a relationship between input intensity and output intensity of a display medium with a typical gamma characteristic. As is well known to those of ordinary skill in the art, the term gamma characteristic is used to describe the non-linear relationship between the input intensity of the display medium and its actual output value. Typically, the display characteristic of the display medium is a curve such as a dotted curve 210 shown in FIG. 2. The dithered display characteristic is then a polygonal line such as a bold line 220 composed of a plurality of linear line segments shown in FIG. 2. The points where the bold line 220 intersects the dotted curve 210 are the intensities that the display medium is capable of displaying without the need to utilize a dithering operation. According to FIG. 2, it is obvious that the output characteristic represented by the bold line 220 that is obtained by the conventional linear dithering operations cannot accurately enough reflect the non-linear characteristic (i.e., the dotted curve 210) of the display medium. This is especially significant along the curvature of the dotted curve 210 where it is greater than the others such as the range from input intensity 251 through 255.